Network Working Group M. Douglass
Internet-Draft Spherical Cow Group
Intended status: Standards Track C. Daboo
Expires: December 31, 2015 Apple
June 29, 2015
Time Zone Data Distribution Servicedraft-ietf-tzdist-service-09
Abstract
This document defines a time zone data distribution service that
allows reliable, secure and fast delivery of time zone data and leap
second rules to client systems such as calendaring and scheduling
applications or operating systems.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 31, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Douglass & Daboo Expires December 31, 2015 [Page 1]

Internet-Draft TZDIST Service June 20151. Introduction
Time zone data typically combines a coordinated universal time (UTC)
offset with daylight saving time (DST) rules. Time zones are
typically tied to specific geographic and geopolitical regions.
Whilst the UTC offset for particular regions changes infrequently,
DST rules can change frequently and sometimes with very little notice
(maybe hours before a change comes into effect).
Calendaring and scheduling systems, such as those that use iCalendar
[RFC5545], as well as operating systems, critically rely on time zone
data to determine the correct local time. As such they need to be
kept up to date with changes to time zone data. To date there has
been no fast and easy way to do that. Time zone data is often
supplied in the form of a set of data files that have to be
"compiled" into a suitable database format for use by the client
application or operating system. In the case of operating systems,
often those changes only get propagated to client machines when there
is an operating system update, which can be infrequent, resulting in
inaccurate time zone data being present for significant amounts of
time. In some cases, old versions of operating systems stop being
supported, but are still in use and thus require users to manually
"patch" their system to keep up to date with time zone changes.
Along with time zone data, it is also important to track the use of
leap seconds to allow a mapping between International Atomic Time
(TAI) and UTC. Leap seconds can be added (or possibly removed) at
various times of year in an irregular pattern typically determined by
precise astronomical observations. The insertion of leap seconds
into UTC is currently the responsibility of the International Earth
Rotation Service.
This specification defines a time zone data distribution service
protocol that allows for fast, reliable and accurate delivery of time
zone data and leap second information to client systems. This
protocol is based on HTTP [RFC7230] using a simple JSON [RFC7159]
based API.
This specification does not define the source of the time zone data
or leap second information. It is assumed that a reliable and
accurate source is available. One such source is the IANA hosted
time zone database [RFC6557].
Discussion of this document has taken place on the tzdist working
group mailing list <tzdist@ietf.org>.
Douglass & Daboo Expires December 31, 2015 [Page 4]

Internet-Draft TZDIST Service June 2015
There can be many contributors. Note this specification does not
address how contributions are made.
(b) Publishers: Publishers aggregate information from contributors,
determine the reliability of the information and, based on that,
generate time zone data. There can be many publishers, each
getting information from many different contributors. In some
cases a publisher may choose to "re-publish" data from another
publisher.
(c) Root Providers: Servers which obtain and then provide the time
zone data from publishers and make that available to other servers
or clients. There can be many root providers. Root providers can
choose to supply time zone data from one or more publishers.
(d) Secondary Providers: Servers which handle the bulk of the
requests and reduce the load on root servers. These will
typically be simple caches of the root server, located closer to
clients. For example a large Internet Service Provider (ISP) may
choose to setup their own secondary provider to allow clients
within their network to make requests of that server rather than
making requests of servers outside their network. Secondary
servers will cache and periodically refresh data from the root
servers.
(e) Clients: Applications, operating systems etc., that make use of
time zone data and retrieve that from either root or secondary
providers.
Some of those layers may be coalesced by implementors. For example,
a vendor may choose to implement the entire service as a single
monolithic virtual server with the address embedded in distributed
systems. Others may choose to provide a service consisting of
multiple layers of providers, many secondary servers and a small
number of root servers.
This specification is concerned only with the protocol used to
exchange data between providers and from provider to client. This
specification does not define how contributors pass their information
to publishers, nor how those publishers vet that information to
obtain trustworthy data, nor the format of the data produced by the
publishers.
3. General Considerations
This section defines several terms and explains some key concepts
used in this specification.
Douglass & Daboo Expires December 31, 2015 [Page 6]

Internet-Draft TZDIST Service June 20153.1. Time Zone
A description of the past and predicted future timekeeping practices
of a collection of clocks that are intended to agree.
Note that the term "time zone" does not have the common meaning of a
region of the world at a specific UTC offset, possibly modified by
daylight saving time. For example, the "Central European Time" zone
can correspond to several time zones "Europe/Berlin", "Europe/Paris",
etc., because subregions have kept time differently in the past.
3.2. Time Zone Data
Data that defines a single time zone, including an identifier, UTC
offset values, DST rules, and other information such as time zone
abbreviations.
3.3. Time Zone Meta-Data
Data that describes additional properties of a time zone that is not
itself included in the time zone data. This can include such things
as the publisher name, version identifier, aliases, and localized
names (see below).
3.4. Time Zone Data Server
A server implementing the Time Zone Data Distribution Service
Protocol defined by this specification.
3.5. Observance
A time zone with varying rules for the UTC offset will have adjacent
periods of time that use different UTC offsets. Each period of time
with a constant UTC offset is called an observance.
3.6. Time Zone Identifiers
Time zone identifiers are unique names associated with each time
zone, as defined by publishers. The iCalendar [RFC5545]
specification has a "TZID" property and parameter whose value is set
to the corresponding time zone identifier, and used to identify time
zone data and relate time zones to start and end dates in events,
etc. This specification does not define what format of time zone
identifiers should be used. It is possible that time zone
identifiers from different publishers overlap, and there might be a
need for a provider to distinguish those with some form of
"namespace" prefix identifying the publisher. However, development
Douglass & Daboo Expires December 31, 2015 [Page 7]

Internet-Draft TZDIST Service June 2015
of a standard (global) time zone identifier naming scheme is out of
scope for this specification.
3.7. Time Zone Aliases
Time zone aliases map a name onto a time zone identifier. For
example "US/Eastern" is usually mapped on to "America/New_York".
Time zone aliases are typically used interchangeably with time zone
identifiers when presenting information to users.
A time zone data distribution service needs to maintain time zone
alias mapping information, and expose that data to clients as well as
allow clients to query for time zone data using aliases. When
returning time zone data to a client, the server returns the data
with an identifier matching the query, but it can include one or more
additional identifiers in the data to provide a hint to the client
that alternative identifiers are available. For example, a query for
"US/Eastern" could include additional identifiers for "America/
New_York" or "America/Montreal".
The set of aliases may vary depending on whether time zone data is
truncated (see Section 3.9). For example, a client located in the US
state of Michigan may see "US/Eastern" as an alias for "America/
Detroit" whereas a client in the US state of New Jersey may see it as
an alias for "America/New_York", and all three names may be aliases
if time zones are truncated to post-2013 data.
3.8. Time Zone Localized Names
Localized names are names for time zones which can be presented to a
user in their own language. Each time zone may have one or more
localized names associated with it. Names would typically be unique
in their own locale as they might be presented to the user in a list.
Localized names are distinct from abbreviations commonly used for UTC
offsets within a time zone. For example, the time zone "America/
New_York" may have the localized name "Nueva York" in a Spanish
locale, as distinct from the abbreviations "EST" and "EDT" which may
or may not have their own localizations.
A time zone data distribution service might need to maintain
localized name information, for one or more chosen languages, as well
as allow clients to query for time zone data using localized names.
3.9. Truncating Time Zones
Time zone data can contain information about past and future UTC
offsets that may not be relevant for a particular server's intended
clients. For example, calendaring and scheduling clients are likely
Douglass & Daboo Expires December 31, 2015 [Page 8]

Internet-Draft TZDIST Service June 2015
most concerned with time zone data that covers a period for one or
two years in the past on into the future, as users typically create
new events only for the present and future. Similarly, time zone
data might contain a large amount of "future" information about
transitions occurring many decades into the future. Again, clients
might be concerned only with a smaller range into the future, and
data past that point might be unnecessary.
To avoid having to send unnecessary data, servers can choose to
truncate time zone data to a range determined by start and end point
date and time values, and provide only offsets and rules between
those points. If such truncation is done, the server MUST include
the ranges it is using in the "capabilities" action response (see
Section 6.1), so that clients can take appropriate action if they
need time zone data for times outside of those ranges.
The truncation points at the start and end of a range are always a
UTC date-time value, with the start point being "inclusive" to the
overall range, and the end point being "exclusive" to the overall
range (i.e., the end value is just past the end of the last valid
value in the range). A server will advertise a truncation range for
the truncated data it can supply, or provide an indicator that it can
truncate at any start or end point to produce arbitrary ranges. In
addition, the server can advertise that it supplies untruncated data
- that is data that covers the full range of times available from the
source publisher. In the absence of any indication of truncated data
available on the server, the server will supply only untruncated
data.
When truncating the start of a "VTIMEZONE" component, the server MUST
include exactly one "STANDARD" or "DAYLIGHT" sub-component with a
"DTSTART" property value that matches the start point of the
truncation range, and appropriate "TZOFFSETFROM" and "TZOFFSETTO"
properties to indicate the correct offset in effect right before and
after the truncation range start point. This sub-component, which is
the first observance defined by the time zone data, represents the
earliest valid date-time covered by the time zone data in the
truncated "VTIMEZONE" component.
When truncating the end of a "VTIMEZONE" component, the server MUST
include a "TZUNTIL" iCalendar property (Section 7.1) in the
"VTIMEZONE" component to indicate the end point of the truncation
range.
Douglass & Daboo Expires December 31, 2015 [Page 9]

Internet-Draft TZDIST Service June 20153.10. Time Zone Versions
Time zone data changes over time and it is important for consumers of
that data to stay up to date with the latest versions. As a result
it is useful to identify individual time zones with a specific
version number or version identifier as supplied by the time zone
data publisher. There are two common models which time zone data
publishers might use to publish updates to time zone data:
a. with the "monolithic" model, the data for all time zones is
published in one go, with a single version number or identifier
applied to the entire data set. e.g., a publisher producing data
several times a year might use version identifiers "2015a",
"2015b", etc.
b. with the "incremental" model, each time zone has its own version
identifier, so that each time zone can be independently updated
without impacting any others. e.g., if the initial data has
version "A.1" for time zone "A", and "B.1" for time zone "B", and
then time zone "B" changes; when the data is next published, time
zone "A" will still have version "A.1", but time zone "B" will
now have "B.2".
A time zone data distribution service needs to ensure that the
version identifiers used by the time zone data publisher are
available to any client, along with the actual publisher name on a
per-time zone basis. This allows clients to compare publisher/
version details on any server, with existing locally cached client
data, and only fetch those time zones which have actually changed
(see Section 4.2.2 for more details on how clients synchronize data
from the server).
4. Time Zone Data Distribution Service Protocol4.1. Server Protocol
The time zone data distribution service protocol uses HTTP [RFC7230]
for query and delivery of time zone data and meta-data, and leap
second information. The interactions with the HTTP server can be
broken down into a set of "actions" that define the overall function
being requested (see Section 5). Each action targets a specific HTTP
resource using the GET method, with various request-URI parameters
altering the behavior as needed.
The HTTP resources used for requests will be identified via URI
templates [RFC6570]. The overall time zone distribution service has
a "context path" request-URI defined as "{/service-prefix}". This
"root" prefix is discovered by the client as per Section 4.2.1.
Douglass & Daboo Expires December 31, 2015 [Page 10]

Internet-Draft TZDIST Service June 2015
Request-URIs that target time zone data directly use the prefix
"{/service-prefix,data-prefix}". The second component of the prefix
template can be used to introduce additional path segments in the
request-URI to allow for alternative ways to "partition" the time
zone data. For example, time zone data might be partitioned by
publisher release dates, or version identifiers. This specification
does not define any partitions, which is left for future extensions.
When the "data-prefix" variable is empty, the server is expected to
return the current version of time zone data it has for all
publishers it supports.
All template-URI variable values, and URI request parameters that
contain text values, MUST be encoded using the UTF-8 [RFC3629]
character set. All responses MUST return data using the UTF-8
[RFC3629] character set. It is important to note that any "/"
characters, which are frequently found in time zone identifiers, are
percent-encoded when used in the value of a path segment expansion
variable in a URI template (as per Section 3.2.6 of [RFC6570]). Thus
the time zone identifier "America/New_York" would appear as
"America%2FNew_York" when used as the value for the "{/tzid}" URI
template variable defined later in this specification.
The server provides time zone meta-data in the form of a JSON
[RFC7159] object. Clients can directly request the time zone meta-
data, or issues queries for subsets of meta-data that match specific
criteria.
Security and privacy considerations for this protocol are discussed
in detail in Section 8 and Section 9, respectively.
4.1.1. Time Zone Queries
Time zone identifiers, aliases or localized names can be used to
query for time zone data or meta-data. This will be more explicitly
defined below for each action. In general however, if a "tzid" URI
template variable is used, then the value may be an identifier or an
alias. When the "pattern" URI query parameter is used it may be an
identifier, an alias or a localized name.
4.1.2. Time Zone Formats
The default media type [RFC2046] format for returning time zone data
is the iCalendar [RFC5545] data format. In addition, the iCalendar-
in-XML [RFC6321], and iCalendar-in-JSON [RFC7265] representations are
also available. Clients use the HTTP Accept header field (see
Section 5.3.2 of [RFC7231]) to indicate their preference for the
returned data format. Servers indicate the available formats that
they support via the "capabilities" action response (Section 5.1).
Douglass & Daboo Expires December 31, 2015 [Page 11]

Internet-Draft TZDIST Service June 20154.1.3. Time Zone Localization
As per Section 3.8, time zone data can support localized names.
Clients use the HTTP Accept-Language header field (see Section 5.3.5
of [RFC7231]) to indicate their preference for the language used for
localized names in the response data.
4.1.4. Conditional Time Zone Requests
When time zone data or meta-data changes, it needs to be distributed
in a timely manner because changes to local time offsets might occur
within a few days of the publication of the time zone data changes.
Typically, the number of time zones that change is small, whilst the
overall number of time zones can be large. Thus, when a client is
using more than a few time zones, it is more efficient for the client
to be able to download only those time zones that have changed (an
incremental update).
Clients initially request a full list of time zones from the server
using a "list" action request (see Section 5.2). The response to
that request includes two items the client caches for use with
subsequent "conditional" (incremental update) requests:
1. An opaque synchronization token in the "synctoken" JSON member.
This token changes whenever there is a change to any meta-data
associated with one or more time zones (where the meta-data is
the information reported in the "list" action response for each
time zone).
2. The HTTP ETag header field value for each time zone returned in
the response. The ETag header field value is returned in the
"etag" JSON member, and corresponds to the ETag header field
value that would be returned when executing a "get" action
request (see Section 5.3) against the corresponding time zone
data resource.
For subsequent updates to cached data, clients can use the following
procedure:
a. Send a "list" action request with a "changedsince" URI query
parameter with its value set to the last opaque synchronization
token returned by the server. The server will return time zone
meta-data for only those time zones that have changed since the
last request.
b. The client will cache the new opaque synchronization token
returned in the response for the next incremental update, along
with the returned time zone meta-data information.
Douglass & Daboo Expires December 31, 2015 [Page 12]

Internet-Draft TZDIST Service June 2015
c. The client will check each time zone meta-data to see if the
"etag" value is different from that of any cached time zone data
it has.
d. The client will use "get" action request to update any cached
time zone data for those time zone's whose ETag header field
value has changed.
Note that time zone meta-data will always change when the
corresponding time zone data changes. However, the converse is not
true: it is possible for some piece of the time zone meta-data to
change without the corresponding time zone data changing. e.g., for
the case of a "monolithic" publisher (see Section 3.10), the version
identifier in every time zone meta-data element will change with each
new published revision, however, only a small subset of time zone
data will actually change.
If a client needs data for only one, or a small set of time zones
(e.g., a clock in a fixed location), then it can use a conditional
HTTP request to determine if the time zone data has changed and
retrieve the new data. The full details of HTTP conditional requests
are described in [RFC7232], what follows is a brief summary of what a
client typically does.
a. When the client retrieves the time zone data from the server
using a "get" action (see Section 5.3) the server will include an
HTTP ETag header field in the response.
b. The client will store the value of that header field along with
the request-URI used for the request.
c. When the client wants to check for an update, it issues another
"get" action HTTP request on the original request-URI, but this
time it includes an If-None-Match HTTP request header field, with
a value set to the ETag header field value from the previous
response. If the data for the time zone has not changed, the
server will return a 304 (Not Modified) HTTP response. If the
data has changed, the server will return a normal HTTP success
response which will include the changed data, as well as a new
value for the ETag header field.
Clients SHOULD poll for changes, using an appropriate conditional
request, at least once a day. A server acting as a secondary
provider, caching time zone data from another server, SHOULD poll for
changes once per hour. See Section 8 on expected client and server
behavior regarding high request rates.
Douglass & Daboo Expires December 31, 2015 [Page 13]

Internet-Draft TZDIST Service June 20154.1.5. Expanded Time Zone Data
Determining time zone offsets at a particular point in time is often
a complicated process, as the rules for daylight saving time can be
complex. To help with this, the time zone data distribution service
provides an action that allows clients to request the server to
expand a time zone into a set of "observances" over a fixed period of
time (see Section 5.4). Each of these observances describes a UTC
onset time and UTC offsets for the prior time and the observance
time. Together, these provide a quick way for "thin" clients to
determine an appropriate UTC offset for an arbitrary date without
having to do full time zone expansion themselves.
4.1.6. Server Requirements
To enable a simple client implementation, servers SHOULD ensure that
they provide or cache data for all commonly used time zones, from
various publishers. That allows client implementations to configure
a single server to get all time zone data. In turn, any server can
refresh any of the data from any other server - though the root
servers may provide the most up-to-date copy of the data.
4.1.7. Error Responses
When an HTTP error response is returned to the client, the server
SHOULD return a JSON "problem detail" object in the response body, as
per [I-D.ietf-appsawg-http-problem]. Every JSON "problem detail"
object MUST include a "type" member with a uri value matching the
applicable error code (defined for each action in Section 5).
4.1.8. Extensions
This protocol is designed to be extensible through a standards based
registration mechanism (see Section 10). It is anticipated that
other useful time zone actions will be added in the future (e.g.,
mapping a geographical location to time zone identifiers, getting
change history for time zones), and so, servers MUST return a
description of their capabilities. This will allow clients to
determine if new features have been installed and, if not, fall back
on earlier features or disable some client capabilities.
4.2. Client Guidelines4.2.1. Discovery
Client implementations need to either know where the time zone data
distribution service is located or discover it through some
mechanism. To use a time zone data distribution service, a client
Douglass & Daboo Expires December 31, 2015 [Page 14]

Internet-Draft TZDIST Service June 2015
needs a fully qualified domain name (FQDN), port and HTTP request-URI
path. The request-URI path found via discovery is the "context path"
for the service itself. The "context path" is used as the value of
the "service-prefix" URI template variable when executing actions
(see Section 5).
The following sub-sections describe two methods of service discovery
using DNS SRV records [RFC2782] and an HTTP "well-known" [RFC5785]
resource. However, alternative mechanisms could also be used (e.g.,
a DHCP server option [RFC2131]).
4.2.1.1. Time Zone Data Distribution Service SRV Service Labels
[RFC2782] defines a DNS-based service discovery protocol that has
been widely adopted as a means of locating particular services within
a local area network and beyond, using SRV RR records. This can be
used to discover a service's FQDN and port.
This specification adds two service types for use with SRV records:
timezone: Identifies a Time Zone Data Distribution server that uses
HTTP without transport layer security ([RFC2818]).
timezones: Identifies a Time Zone Data Distribution server that uses
HTTP with transport layer security ([RFC2818]).
Clients MUST honor "TTL", "Priority" and "Weight" values in the SRV
records, as described by [RFC2782].
Example: service record for server without transport layer security.
_timezone._tcp SRV 0 1 80 tz.example.com.
Example: service record for server with transport layer security.
_timezones._tcp SRV 0 1 443 tz.example.com.
4.2.1.2. Time Zone Data Distribution Service TXT records
When SRV RRs are used to advertise a time zone data distribution
service, it is also convenient to be able to specify a "context path"
in the DNS to be retrieved at the same time. To enable that, this
specification uses a TXT RR that follows the syntax defined in
Section 6 of [RFC6763] and defines a "path" key for use in that
record. The value of the key MUST be the actual "context path" to
the corresponding service on the server.
Douglass & Daboo Expires December 31, 2015 [Page 15]

Internet-Draft TZDIST Service June 2015
A site might provide TXT records in addition to SRV records for each
service. When present, clients MUST use the "path" value as the
"context path" for the service in HTTP requests. When not present,
clients use the ".well-known" URI approach described in
Section 4.2.1.3.
To facilitate "context path's" that might differ from user to user,
the server MAY require authentication when a client tries to access
the path URI specified by the TXT RR (i.e., the server would return a
401 status response to the unauthenticated request from the client,
then return a redirect response after a successful authentication by
the client).
Example: text record for service with transport layer security.
_timezones._tcp TXT path=/timezones
4.2.1.3. Time Zone Data Distribution Service Well-Known URI
A "well-known" URI [RFC5785] is registered by this specification for
the Time Zone Data Distribution service, "timezone" (see Section 10).
This URI points to a resource that the client can use as the initial
"context path" for the service they are trying to connect to. The
server MUST redirect HTTP requests for that resource to the actual
"context path" using one of the available mechanisms provided by HTTP
(e.g., using an appropriate 3xx status response). Clients MUST
handle HTTP redirects on the ".well-known" URI. Servers MUST NOT
locate the actual time zone data distribution service endpoint at the
".well-known" URI as per Section 1.1 of [RFC5785]. The "well-known"
URI MUST be present on the server, even when a TXT RR
(Section 4.2.1.2) is used in the DNS to specify a "context path".
Servers SHOULD set an appropriate Cache-Control header field value
(as per Section 5.2 of [RFC7234]) in the redirect response to ensure
caching occurs as needed, or as required by the type of response
generated. For example, if it is anticipated that the location of
the redirect might change over time, then an appropriate "max-age"
value would be used.
To facilitate "context path's" that might differ from user to user,
the server MAY require authentication when a client tries to access
the ".well-known" URI (i.e., the server would return a 401 status
response to the unauthenticated request from the client, then return
the redirect response after a successful authentication by the
client).
Douglass & Daboo Expires December 31, 2015 [Page 16]

Internet-Draft TZDIST Service June 20154.2.1.3.1. Example: well-known URI redirects to actual context path
A Time Zone Data Distribution server has a "context path" that is
"/servlet/timezone". The client will use "/.well-known/timezone" as
the path for the service after it has first found the FQDN and port
number via an SRV lookup or via manual entry of information by the
user. When the client makes its initial HTTP request against
"/.well-known/timezone", the server would issue an HTTP 301 redirect
response with a Location response header field using the path
"/servlet/timezone". The client would then "follow" this redirect to
the new resource and continue making HTTP requests there. The client
would also cache the redirect information, subject to any Cache-
Control directive, for use in subsequent requests.
4.2.2. Synchronization of Time Zones
This section discusses possible client synchronization strategies
using the various protocol elements provided by the server for that
purpose.
4.2.2.1. Initial Synchronization of All Time Zones
When a secondary service or a client wishing to cache all time zone
data first starts, or wishes to do a full refresh, it synchronizes
with another server by first issuing a "list" action to retrieve all
the time zone meta-data. The client would preserve the returned
opaque token for subsequent use (see "synctoken" in Section 5.2.1).
The client will store the meta-data for each time zone returned in
the response. Time zone data for each corresponding time zone can
then be fetched and stored locally. In addition a mapping of aliases
to time zones can be built from the meta-data.
4.2.2.2. Subsequent Synchronization of All Time Zones
A secondary service or a client caching all time zones needs to
periodically synchronize with a server. To do so it would issue a
"list" action with the "changedsince" URI query parameter set to the
value of the opaque token returned by the last synchronization. The
client would again preserve the returned opaque token for subsequent
use. The client will update its stored time zone meta-data using the
new values returned in the response, which contains just the time
zone meta-data for those time zones changed since the last
synchronization. In addition, it will compare the "etag" value in
each time zone meta-data to the ETag header field value for the
corresponding time zone data resource it has previously cached, and
if different, it will fetch the new time zone data. Note that if the
client presents the server with a "changedsince" value that the
server does not support, all time zone data will be returned, as it
Douglass & Daboo Expires December 31, 2015 [Page 17]

Internet-Draft TZDIST Service June 2015
would for the case where the request did not include a "changedsince"
value.
Publishers should take into account the fact that the "outright"
deletion of time zone names will cause problems to simple clients and
so aliasing a deleted time zone identifier to a suitable alternate
one is preferable.
4.2.2.3. Synchronization with Pre-Existing Time Zone Data
A client might be pre-provisioned with time zone data from a source
other than the time zone data distribution service it is configured
to use. In such cases, the client might want to minimize the amount
of time zone data it synchronizes by doing an initial "list" action
to retrieve all the time zone meta-data, but then only fetch time
zone data for those time zones that do not match the publisher and
version details for the pre-provisioned data.
5. Actions
Servers MUST support the following actions. The information below
shows details about each action: the request-URI the client targets
(in the form of a URI template [RFC6570]) a description, the set of
allowed query parameters, the nature of the response, and a set of
possible error codes for the response (see Section 4.1.7).
For any error not covered by the specific error codes defined below,
the "urn:ietf:params:tzdist:error:invalid-action" error code is
returned to the client in the JSON "problem details" object.
The examples in the following subsections presume that the timezone
context path has been discovered to be "/servlet/timezone" (as in the
example in Section 4.2.1.3.1).
5.1. "capabilities" Action
Name: capabilities
Request-URI Template:
{/service-prefix}/capabilities
Description: This action returns the capabilities of the server,
allowing clients to determine if a specific feature has been
deployed and/or enabled.
Parameters: None
Douglass & Daboo Expires December 31, 2015 [Page 18]

Internet-Draft TZDIST Service June 2015
"multi": false
}
]
},
{
"name": "find",
"uri-template": "/servlet/timezone/zones{?pattern}",
"parameters": [
{
"name": "pattern",
"required": true,
"multi": false
}
]
},
{
"name": "leapseconds",
"uri-template": "/servlet/timezone/leapseconds",
"parameters": []
}
]
}
5.2. "list" Action
Name: list
Request-URI Template:
{/service-prefix,data-prefix}/zones{?changedsince}
Description: This action lists all time zone identifiers in summary
format, with publisher, version, aliases and optional localized
data. In addition, it returns an opaque synchronization token for
the entire response. If the "changedsince" URI query parameter is
present, its value MUST correspond to a previously returned
synchronization token value. When "changedsince" is used, the
server MUST return only those time zones that have changed since
the specified synchronization token. If the "changedsince" value
is not supported by the server, the server MUST return all time
zones, treating the request as if it had no "changedsince".
Parameters:
changedsince OPTIONAL, and MUST NOT occur more than once.
Douglass & Daboo Expires December 31, 2015 [Page 21]

Internet-Draft TZDIST Service June 20155.3. "get" Action
Name: get
Request-URI Template:
{/service-prefix,data-prefix}/zones{/tzid}{?start,end}
The "tzid" variable value is REQUIRED in order to distinguish this
action from the "list" action.
Description: This action returns a time zone. The response MUST
contain an ETag response header field indicating the current value
of the strong entity tag of the time zone resource.
In the absence of any Accept HTTP request header field, the server
MUST return time zone data with the "text/calendar" media type.
If the "tzid" variable value is actually a time zone alias, the
server will return the matching time zone data with the alias as
the identifier in the time zone data. The server MAY include one
or more "TZID-ALIAS-OF" properties (see Section 7.2) in the time
zone data to indicate additional identifiers that have the
matching time zone identifier as an alias.
Parameters:
start=<date-time> OPTIONAL, and MUST NOT occur more than once.
Specifies the inclusive UTC date-time value at which the
returned time zone data is truncated at its start.
end=<date-time> OPTIONAL, and MUST NOT occur more than once.
Specifies the exclusive UTC date-time value at which the
returned time zone data is truncated at its end.
Response: A document containing all the requested time zone data in
the format specified.
Possible Error Codes
urn:ietf:params:tzdist:error:tzid-not-found No time zone
associated with the specified "tzid" path segment value was
found.
urn:ietf:params:tzdist:error:invalid-format The Accept request
header field supplied by the client did not contain a media
type for time zone data supported by the server.
Douglass & Daboo Expires December 31, 2015 [Page 23]

Internet-Draft TZDIST Service June 2015
urn:ietf:params:tzdist:error:invalid-start The "start" URI query
parameter has an incorrect value, or appears more than once, or
does not match one of the fixed truncation range start values
advertised in the "capabilities" action response.
urn:ietf:params:tzdist:error:invalid-end The "end" URI query
parameter has an incorrect value, or appears more than once, or
has a value less than or equal to the "start" URI query
parameter, or does not match one of the fixed truncation range
end values advertised in the "capabilities" action response.
5.3.1. Example: Get time zone data
In this example the client requests the time zone with a specific
time zone identifier to be returned.
>> Request <<
GET /servlet/timezone/zones/America%2FNew_York HTTP/1.1
Host: tz.example.com
Accept:text/calendar
>> Response <<
HTTP/1.1 200 OK
Date: Wed, 4 Jun 2008 09:32:12 GMT
Content-Type: text/calendar; charset="utf-8"
Content-Length: xxxx
ETag: "123456789-000-111"
BEGIN:VCALENDAR
...
BEGIN:VTIMEZONE
TZID:America/New_York
...
END:VTIMEZONE
END:VCALENDAR
5.3.2. Example: Conditional Get time zone dataDouglass & Daboo Expires December 31, 2015 [Page 24]

Internet-Draft TZDIST Service June 2015
In this example the client requests the time zone with a specific
time zone identifier to be returned, but uses an If-None-Match header
field in the request, set to the value of a previously returned ETag
header field, or the value of the "etag" member in a JSON "timezone"
object returned from a "list" action response. In this example, the
data on the server has not changed, so a 304 response is returned.
>> Request <<
GET /servlet/timezone/zones/America%2FNew_York HTTP/1.1
Host: tz.example.com
Accept:text/calendar
If-None-Match: "123456789-000-111"
>> Response <<
HTTP/1.1 304 Not Modified
Date: Wed, 4 Jun 2008 09:32:12 GMT
5.3.3. Example: Get time zone data using a time zone aliasDouglass & Daboo Expires December 31, 2015 [Page 25]

Internet-Draft TZDIST Service June 2015
In this example the client requests the time zone with a specific
time zone identifier truncated at one of the ranges specified by the
server, to be returned. Note the presence of a "STANDARD" component
that matches the start point of the truncation range (converted to
the local time for the UTC offset in effect at the matching UTC
time). Also, note the presence of the "TZUNTIL" (Section 7.1)
iCalendar property in the "VTIMEZONE" component, indicating the upper
bound on the validity of the time zone data.
>> Request <<
GET /servlet/timezone/zones/America%2FNew_York
?start=2010-01-01T00:00:00Z&end=2020-01-01T00:00:00Z HTTP/1.1
Host: tz.example.com
Accept:text/calendar
>> Response <<
HTTP/1.1 200 OK
Date: Wed, 4 Jun 2008 09:32:12 GMT
Content-Type: text/calendar; charset="utf-8"
Content-Length: xxxx
ETag: "123456789-000-111"
BEGIN:VCALENDAR
...
BEGIN:VTIMEZONE
TZID:America/New_York
TZUNTIL:20200101T000000Z
BEGIN:STANDARD
DTSTART:20101231T190000
TZNAME:EST
TZOFFSETFROM:-0500
TZOFFSETTO:-0500
END:STANDARD
...
END:VTIMEZONE
END:VCALENDAR
5.3.5. Example: Request for a non-existent time zoneDouglass & Daboo Expires December 31, 2015 [Page 27]

Internet-Draft TZDIST Service June 2015
In this example the client requests the time zone with a specific
time zone identifier to be returned. As it turns out, no time zone
exists with that identifier.
>> Request <<
GET /servlet/timezone/zones/America%2FPittsburgh HTTP/1.1
Host: tz.example.com
Accept:application/calendar+json
>> Response <<
HTTP/1.1 404 Not Found
Date: Wed, 4 Jun 2008 09:32:12 GMT
Content-Type: application/problem+json; charset="utf-8"
Content-Language: en
Content-Length: xxxx
{
"type": "urn:ietf:params:tzdist:error:tzid-not-found",
"title": "Time zone identifier was not found on this server",
"status": 404
}
5.4. "expand" Action
Name: expand
Request-URI Template:
{/service-prefix,data-prefix}/zones{/tzid}/observances{?start,end}
The "tzid" variable value is REQUIRED.
Description: This action expands the specified time zone into a list
of onset start date/time (in UTC) and UTC offsets. The response
MUST contain an ETag response header field indicating the current
value of the strong entity tag of the time zone being expanded.
Parameters:
start=<date-time>: REQUIRED, and MUST occur only once. Specifies
the inclusive UTC date-time value for the start of the period
of interest.
end=<date-time>: REQUIRED, and MUST occur only once. Specifies
the exclusive UTC date-time value for the end of the period of
interest. Note that this is the exclusive end value - i.e., it
represents the date just after the range of interest. e.g., if
Douglass & Daboo Expires December 31, 2015 [Page 28]

Internet-Draft TZDIST Service June 2015
a client wants the expanded date just for the year 2014, it
would use a start value of "2014-01-01T00:00:00Z" and an end
value of "2015-01-01T00:00:00Z". An error occurs if the end
value is less than or equal to the start value.
Response: A JSON object containing a "tzid" member, and an
"observances" member, see Section 6.3. If the time zone being
expanded is not fully defined over the requested time range (e.g.,
because of truncation), then the server MUST include "start" and/
or "end" members in the JSON response to indicate the actual start
and end point for the observances being returned. The server MUST
include an expanded observance representing the time zone
information in effect at the start of the returned observance
period.
Possible Error Codes
urn:ietf:params:tzdist:error:tzid-not-found No time zone
associated with the specified "tzid" path segment value was
found.
urn:ietf:params:tzdist:error:invalid-start The "start" URI query
parameter has an incorrect value, or appears more than once, or
is missing, or has a value outside any fixed truncation ranges
advertised in the "capabilities" action response.
urn:ietf:params:tzdist:error:invalid-end The "end" URI query
parameter has an incorrect value, or appears more than once, or
has a value less than or equal to the "start" URI query
parameter, or has a value outside any fixed truncation ranges
advertised in the "capabilities" action response..
5.4.1. Example: Expanded JSON Data FormatDouglass & Daboo Expires December 31, 2015 [Page 29]

Internet-Draft TZDIST Service June 2015
Description: This action allows a client to query the time zone data
distribution service for a matching identifier, alias or localized
name, using a simple "glob" style pattern match against the names
known to the server (with an asterisk * as the wildcard
character). Pattern match strings (which have to be %-encoded and
then decoded when used in the URI query parameter) have the
following options:
* not present: An exact text match is done, e.g., "xyz"
* first character only: An ends-with text match is done, e.g.,
"*xyz"
* last character only: A starts-with text match is done, e.g.,
"xyz*"
* first and last characters only: A sub-string text match is
done, e.g., "*xyz*"
Escaping \ and *: To match 0x2A ("*") and 0x5C ("\") characters
in a time zone identifier, those characters have to be
"escaped" in the pattern by prepending a single 0x5C
("\")character. e.g., a pattern "\*Test\\Time\*Zone\*" is used
for an exact match against the time zone identifier
"*Test\Time*Zone*". An unescaped "*" character MUST NOT appear
in the middle of the string and MUST result in an error. An
unescaped "\" character MUST NOT appear anywhere in the string
and MUST result in an error.
In addition, when matching:
Underscores: Underscore characters (0x5F) in time zone
identifiers MUST be mapped to a single space character (0x20)
prior to string comparison in both the pattern and time zone
identifiers being matched. This allows time zone identifiers
such as "America/New_York" to match a query for "*New York*".
Case mapping: ASCII characters in the range 0x41 ("A") through
0x5A ("Z") MUST be mapped to their lowercase equivalents in
both the pattern and time zone identifiers being matched.
Parameters:
pattern=<text> REQUIRED, and MUST occur only once.
Response: The response has the same format as the "list" action,
with one result object per successful match, see Section 6.2.
Douglass & Daboo Expires December 31, 2015 [Page 31]

Internet-Draft TZDIST Service June 2015
Possible Error Codes
urn:ietf:params:tzdist:error:invalid-pattern The "pattern" URI
query parameter has an incorrect value, or appears more than
once.
5.5.1. Example: Find action
In this example the client asks for data about the time zone "US/
Eastern".
Douglass & Daboo Expires December 31, 2015 [Page 32]

Internet-Draft TZDIST Service June 20155.6. "leapseconds" Action
Name: leapseconds
Request-URI Template:
{/service-prefix,data-prefix}/leapseconds
Description: This action allows a client to query the time zone data
distribution service to retrieve the current leap second
information available on the server.
Parameters: None
Response: A JSON object containing an "expires" member, a
"publisher" member, a "version" member, and a "leapseconds"
member, see Section 6.4. The "expires" member in the JSON
response indicates the latest date covered by leap second
information. e.g., (from the example below) if the "expires" value
is set to "2014-06-28" and the latest leap second change indicated
was at "2012-07-01", then the data indicates that there are no
leap seconds added (or removed) between those two dates, and
information for leap seconds beyond the "expires" date is not yet
available.
The "leapseconds" member contains a list of JSON objects each of
which contains a "utc-offset" and "onset" member. The "onset"
member specifies the date (with the implied time of 00:00:00 UTC)
at which the corresponding UTC offset from TAI takes effect. In
other words, a leap second is added or removed just prior to time
00:00:00 UTC of the specified onset date. When a leap second is
added, the "utc-offset" value will be incremented by one, when a
leap second is removed, the "utc-offset" value will be decremented
by one.
Possible Error Codes No specific code.
5.6.1. Example: Get leapsecond informationDouglass & Daboo Expires December 31, 2015 [Page 34]

Internet-Draft TZDIST Service June 2015
OBJECT represents a JSON object, defined in Section 4 of [RFC7159].
"OBJECT" is followed by a parenthesized list of "MEMBER" rule
names. If a member rule name is preceded by a "?" (0x3F)
character, that member is optional, otherwise all members are
required. If two or more member rule names are present, each
separated from the other by a "|" (0x7C) character, then only one
of those members MUST be present in JSON object. JSON object
members are unordered, and thus the order used in the rules is not
significant.
MEMBER represents a member of a JSON object, defined in Section 4 of
[RFC7159]. "MEMBER" is followed by a rule name, then the name of
the member, followed by a ":", and then the value. A value can be
one of "OBJECT", "ARRAY", "NUMBER", "STRING", or "BOOLEAN" rules.
ARRAY represents a JSON array, defined in Section 5 of [RFC7159].
"ARRAY" is followed by a value (one of "OBJECT", "ARRAY",
"NUMBER", "STRING", or "BOOLEAN"), indicating the type of items
used in the array.
NUMBER represents a JSON number, defined in Section 6 of [RFC7159].
STRING represents a JSON string, defined in Section 7 of [RFC7159].
BOOLEAN represents either of the JSON values "true" or "false",
defined in Section 3 of [RFC7159].
; a line starting with a ";" (0x3B) character is a comment.
Note, clients MUST ignore any unexpected JSON members in responses
from the server.
6.1. capabilities action response
Rules for the JSON document returned for a "capabilities" action
request.
; root object
OBJECT (version, info, actions)
; The version number of the protocol supported - MUST be 1
MEMBER version "version" : NUMBER
; object containing service information
; Only one of primary_source or secondary_source MUST be present
MEMBER info "info" : OBJECT (
primary_source | secondary_source,
formats,
Douglass & Daboo Expires December 31, 2015 [Page 36]

Internet-Draft TZDIST Service June 2015
?truncated,
?provider_details,
?contacts
)
; The source of the time zone data provided by a "primary" server
MEMBER primary_source "primary-source" : STRING
; The time zone data server from which data is provided by a
; "secondary" server
MEMBER secondary_source "secondary-source" : STRING
; Array of one or more media types for the time zone data formats
; that the server can return
MEMBER formats "formats" : ARRAY STRING
; Present if the server is providing truncated time zone data. The
; value is an object providing details of the supported truncation
; modes.
MEMBER truncated "truncated" : OBJECT: (
any,
?ranges,
?untruncated
)
; Indicates whether the server can truncate time zone data at any
; start or end point. When set to "true" any start or end point is
; a valid value for use with the "start" and "end" URI query
; parameters in a "get" action request
MEMBER any "any" : BOOLEAN
; Indicates which ranges of time the server has truncated data for.
; A value from this list may be used with the "start" and "end" URI
; query parameters in a "get" action request. Not present if "any"
; is set to "true"
MEMBER ranges "ranges" : ARRAY OBJECT (range-start, range-end)
; [RFC3339] UTC date-time value for inclusive start of the range,
; or the single character "*" to indicate a value corresponding to
; the lower bound supplied by the publisher of the time zone data
MEMBER range-start "start" : STRING
; [RFC3339] UTC date-time value for exclusive end of the range,
; or the single character "*" to indicate a value corresponding to
; the upper bound supplied by the publisher of the time zone data
MEMBER range-end "end" : STRING
; Indicates whether the server can can supply untruncated data. When
Douglass & Daboo Expires December 31, 2015 [Page 37]

Internet-Draft TZDIST Service June 2015
; set to "true" indicates that, in addition to truncated data being
; available, the server can return untruncated data if a "get"
; action request is executed without a "start" or "end" URI query
; parameter
MEMBER untruncated "untruncated" : BOOLEAN
; A URI where human readable details about the time zone service
; is available
MEMBER provider_details "provider-details" : STRING
; Array of URIs providing contact details for the server
; administrator
MEMBER contacts "contacts" : ARRAY STRING
; Array of actions supported by the server
MEMBER actions "actions" : ARRAY OBJECT (
action_name,
action_params
)
; Name of the action
MEMBER action_name: "name" : STRING
; Array of request-URI query parameters supported by the action
MEMBER action_params: "parameters" ARRAY OBJECT (
param_name,
?param_required,
?param_multi,
?param_values
)
; Name of the parameter
MEMBER param_name "name" : STRING
; If true the parameter has to be present in the request-URI
; default is false
MEMBER param_required "required" : BOOLEAN
; If true the parameter can occur more than once in the request-URI
; default is false
MEMBER param_multi "multi" : BOOLEAN,
; An array that defines the allowed set of values for the parameter
; In the absence of this member, any string value is acceptable
MEMBER param_values "values" ARRAY STRING
Douglass & Daboo Expires December 31, 2015 [Page 38]

Internet-Draft TZDIST Service June 20156.2. list/find action response
Rules for the JSON document returned for a "list" or "find" action
request.
; root object
OBJECT (synctoken, timezones)
; Server generated opaque token used for synchronizing changes,
MEMBER synctoken "synctoken" : STRING
; Array of time zone objects
MEMBER timezones "timezones" : ARRAY OBJECT (
tzid,
etag,
last_modified,
publisher,
version,
?aliases,
?local_names,
)
; Time zone identifier
MEMBER tzid "tzid" : STRING
; Current ETag for the corresponding time zone data resource
MEMBER etag "etag" : STRING
; Date/time when the time zone data was last modified
; [RFC3339] UTC date-time value
MEMBER last_modified "last-modified" : STRING
; Time zone data publisher
MEMBER publisher "publisher" : STRING
; Current version of the time zone data as defined by the
; publisher
MEMBER version "version" : STRING
; An array that lists the set of time zone aliases available
; for the corresponding time zone
MEMBER aliases "aliases" : ARRAY STRING
; An array that lists the set of localized names available
; for the corresponding time zone
MEMBER local_names "local-names" : ARRAY OBJECT (
lname, lang, ?pref
)
Douglass & Daboo Expires December 31, 2015 [Page 39]

Internet-Draft TZDIST Service June 2015
Rules for the JSON document returned for a "expand" action request.
; root object
OBJECT (
tzid,
?start,
?end,
observances
)
; Time zone identifier
MEMBER tzid "tzid" : STRING
; The actual inclusive start point for the returned observances
; if different from the value of the "start" URI query parameter
MEMBER start "start" : STRING
; The actual exclusive end point for the returned observances
; if different from the value of the "end" URI query parameter
MEMBER end "end" : STRING
; Array of time zone objects
MEMBER observances "observances" : ARRAY OBJECT (
oname,
?olocal_names,
onset,
utc_offset_from,
utc_offset_to
)
; Observance name
MEMBER oname "name" : STRING
; Array of localized observance names
MEMBER olocal_names "local-names" : ARRAY STRING
; [RFC3339] UTC date-time value at which the observance takes effect
MEMBER onset "onset" : STRING
; The UTC offset in seconds before the start of this observance
MEMBER utc_offset_from "utc-offset-from" : NUMBER
; The UTC offset in seconds at and after the start of this observance
MEMBER utc_offset_to "utc-offset-to" : NUMBER
Douglass & Daboo Expires December 31, 2015 [Page 41]

Internet-Draft TZDIST Service June 20156.4. leapseconds action response
Rules for the JSON document returned for a "leapseconds" action
request.
; root object
OBJECT (
expires,
publisher,
version,
leapseconds
)
; Last valid date covered by the data in this response
; [RFC3339] full-date value
MEMBER expires "expires" : STRING
; Leap second information publisher
MEMBER publisher "publisher" : STRING
; Current version of the leap second information as defined by the
; publisher
MEMBER version "version" : STRING
; Array of leap second objects
MEMBER leapseconds "leapseconds" : ARRAY OBJECT (
utc_offset,
onset
)
; The UTC offset from TAI in seconds in effect at and after the
; specified date
MEMBER utc_offset "utc-offset" : NUMBER
; [RFC3339] full-date value at which the new UTC offset takes effect,
; at T00:00:00Z
MEMBER onset "onset" : STRING
7. New iCalendar Properties7.1. Time Zone Upper Bound
Property Name: TZUNTIL
Purpose: This property specifies an upper bound for the validity of
data within a "VTIMEZONE" component.
Value Type: DATE-TIME
Douglass & Daboo Expires December 31, 2015 [Page 42]

Internet-Draft TZDIST Service June 2015
Property Parameters: IANA and non-standard property parameters can
be specified on this property.
Conformance: This property can be specified zero or one time within
"VTIMEZONE" calendar components.
Description: The value MUST be specified in the UTC time format.
Time zone data in a "VTIMEZONE" component might cover only a fixed
period of time. The start of such a period is clearly indicated
by the earliest observance defined by the "STANDARD" and
"DAYLIGHT" sub-components. However, [RFC5545] does not define a
way to indicate an upper bound on the validity of the time zone
data, which cannot be simply derived from the observance with the
latest onset time. This specification introduces the "TZUNTIL"
property for that purpose. It specifies an "exclusive" UTC date-
time value that indicates the last time at which the time zone
data is to be considered valid.
This property is also used by time zone data distribution servers
to indicate the truncation range end point of time zone data (as
described in Section 3.9).
Format Definition: This property is defined by the following
notation:
tzuntil = "TZUNTIL" tzuntilparam ":" date-time CRLF
tzuntilparam = *(";" other-param)
Example: Suppose a time zone based on astronomical observations has
well-defined onset times through the year 2025, but the first
onset in 2026 is currently known only approximately. In that
case, the "TZUNTIL" property could be specified as follows:
TZUNTIL:20260101T000000Z
7.2. Time Zone Identifier Alias Property
Property Name: TZID-ALIAS-OF
Purpose: This property specifies a time zone identifier that the
main time zone identifier is an alias of.
Value Type: TEXT
Property Parameters: IANA and non-standard property parameters can
be specified on this property.
Douglass & Daboo Expires December 31, 2015 [Page 43]

Internet-Draft TZDIST Service June 2015
Conformance: This property can be specified zero or more times
within "VTIMEZONE" calendar components.
Description: When the "VTIMEZONE" component uses a time zone
identifier alias for the "TZID" property value, the "TZID-ALIAS-
OF" property is used to indicate the time zone identifier of the
other time zone (see Section 3.7).
Format Definition: This property is defined by the following
notation:
tzid-alias-of = "TZID-ALIAS-OF" tzidaliasofparam ":"
[tzidprefix] text CRLF
tzidaliasofparam = *(";" other-param)
;tzidprefix defined in [RFC5545].
Example: The following is an example of this property:
TZID-ALIAS-OF:America/New_York
8. Security Considerations
Time zone data is critical in determining local or UTC time for
devices and in calendaring and scheduling operations. As such, it is
vital that a reliable source of time zone data is used. Servers
providing a time zone data distribution service MUST support HTTP
over Transport Layer Security (TLS) (as defined by [RFC2818] and
[RFC5246], with best practices described in [RFC7525]). Servers MAY
support a time zone data distribution service over HTTP without TLS.
However, secondary servers MUST use TLS to fetch data from a primary
server.
Clients SHOULD use transport layer security as defined by [RFC2818],
unless they are specifically configured otherwise. Clients that have
been configured to use the TLS-based service, MUST NOT fall back to
using the non-TLS service if the TLS-based service is not available.
In additional, clients MUST NOT follow HTTP redirect requests from a
TLS service to a non-TLS service. When using TLS, clients MUST
verify the identity of the server, using a standard, secure mechanism
such as the certificate verification process specified in [RFC6125]
or DANE [RFC6698].
A malicious attacker with access to the DNS server data, or able to
get spoofed answers cached in a recursive resolver, can potentially
cause clients to connect to any server chosen by the attacker. In
the absence of a secure DNS option, clients SHOULD check that the
Douglass & Daboo Expires December 31, 2015 [Page 44]

Internet-Draft TZDIST Service June 2015
target FQDN returned in the SRV record is the same as the original
service domain that was queried, or is a sub-domain of the original
service domain. In many cases the client configuration is likely to
be handled automatically without any user input and as such, any
mismatch between the original service domain and the target FQDN is
treated as a failure and the client MUST NOT attempt to connect to
the target server. In addition, when transport layer security is
being used, the transport layer security certificate SHOULD include
an SRV-ID field as per [RFC4985] matching the expected DNS SRV
queries clients will use for service discovery. If an SRV-ID field
is present in a certificate, clients MUST match the SRV-ID value with
the service type and domain that matches the DNS SRV request made by
the client to discover the service.
Time zone data servers SHOULD protect themselves against poorly
implemented or malicious clients by throttling high request rates or
frequent requests for large amounts of data. Clients can avoid being
throttled by using the polling capabilities outlined in
Section 4.1.4. Servers MAY require some form of authentication or
authorization of clients (including secondary servers), as per
[RFC7235], to restrict which clients are allowed to access their
service, or provide better identification of problematic clients.
9. Privacy Considerations
The type and pattern of requests that a client makes can be used to
"fingerprint" specific clients or devices and thus potentially used
to track information about what the users of the clients might be
doing. In particular, a client that only downloads time zone data on
an as needed basis, will leak the fact that a user's device has moved
from one time zone to another or that the user is receiving
scheduling messages from another user in a different time zone.
Clients need to be aware of the potential ways in which an untrusted
server or a network observer might be able to track them and take
precautions such as the following:
1. Always use TLS to connect to the server.
2. Avoid use of TLS session resumption.
3. Always fetch and synchronize the entire set of time zone data to
avoid leaking information about which time zones are actually in
use by the client.
4. Randomize the order in which individual time zones are fetched
using the "get" action, when retrieving a set of time zones based
on a "list" action response.
Douglass & Daboo Expires December 31, 2015 [Page 45]

Internet-Draft TZDIST Service June 2015
5. Avoid use of conditional HTTP requests [RFC7232] with the "get"
action to prevent tracking of clients by servers generating
client-specific ETag header field values.
6. Avoid use of cookies in HTTP requests [RFC6265].
7. Avoid use of authenticated HTTP requests.
8. When doing periodic polling to check for updates, apply a random
(positive or negative) offset to the next poll time to avoid
servers being able to identify the client by the specific
periodicity of its polling behavior.
9. A server trying to "fingerprint" clients might insert a "fake"
time zone into the time zone data, using a unique identifier for
each client making a request. The server can then watch for
client requests that refer to that "fake" time zone and thus
track the activity of each client. It is hard for clients to
identify a "fake" time zone given that new time zones are added
from time to time. One option to mitigate this would be for the
client to make use of two time zone distribution servers from two
independent providers, that provide time zone data from the same
publisher. The client can then compare the list of time zones
from each server (assuming they both have the same version of
time zone data from the common publisher) and detect ones that
appear to be added on one server and not the other.
Alternatively, the client can check the publisher data directly
to verify that time zones match the set the publisher has.
Note that some of the above recommendations will result in less
efficient use of the protocol due to fetching data that might not be
relevant to the client.
An organization can setup a secondary server within their own domain,
and configure their clients to use that server, to protect the
organization's users from the possibility of being tracked by an
untrusted time zone distribution server. Clients can then use more
efficient protocol interactions, free from the concerns above, on the
basis that their organization's server is trusted. When doing this,
the secondary server would follow the recommendations for clients
(listed in the previous paragraph) so that the untrusted server is
not able to gain information about the organization as a whole.
Note, however, that if client requests to the secondary server are
subject to tracking by a network observer so clients ought to apply
some of the randomization techniques from the list above.
Servers that want to avoid accidentally storing information that
could be used to identify clients can take the following precautions:
Douglass & Daboo Expires December 31, 2015 [Page 46]

Internet-Draft TZDIST Service June 2015
1. Avoid logging client request activity, or anonymize information
in any logs (e.g., client IP address, client user-agent details,
authentication credentials, etc).
2. Add an unused HTTP response header to each response with a random
amount of data in it (e.g., to pad the overall request size to
the nearest power-of-2 or 128-byte boundary) to avoid exposing
which time zones are being fetched when TLS is being used, via
network traffic analysis.
10. IANA Considerations
This specification defines a new registry of "actions" for the time
zone data distribution service protocol, defines a "well-known" URI
using the registration procedure and template from Section 5.1 of
[RFC5785], creates two new SRV service label aliases, and defines one
new iCalendar property parameter as per the registration procedure in
[RFC5545]. It also adds a new "tzdist Identifiers Registry" to the
IETF parameters URN sub-namespace as per [RFC3553] for use with
protocol related error codes.
10.1. Service Actions Registration
IANA is asked to create a new top-level category called "Time Zone
Distribution Service (TZDIST) Parameters", and to put all the
registries created herein into that category.
IANA is asked to create a new registry called "TZDIST Service
Actions", as defined below.
10.1.1. Service Actions Registration Procedure
This registry uses the "Specification Required" policy defined in
[I-D.leiba-cotton-iana-5226bis], which makes use of a designated
expert to review potential registrations.
The IETF will create a mailing list, tzdist-service@ietf.org, which
can be used for public discussion of time zone data distribution
service actions proposals prior to registration. The IESG will
appoint a designated expert who will monitor the tzdist-
service@ietf.org mailing list and review registrations.
A Standards Track RFC is REQUIRED for changes to actions previously
documented in a Standards Track RFC, otherwise any public
specification that satisfies the requirements of
[I-D.leiba-cotton-iana-5226bis] is acceptable.
Douglass & Daboo Expires December 31, 2015 [Page 47]

Internet-Draft TZDIST Service June 2015
The registration procedure begins when a completed registration
template, as defined below, is sent to tzdist-service@ietf.org and
iana@iana.org. The designated expert is expected to tell IANA and
the submitter of the registration whether the registration is
approved, approved with minor changes, or rejected with cause, within
two weeks. When a registration is rejected with cause, it can be re-
submitted if the concerns listed in the cause are addressed.
Decisions made by the designated expert can be appealed as per
Section 10 of [I-D.leiba-cotton-iana-5226bis].
The designated expert MUST take the following requirements into
account when reviewing the registration:
1. A valid registration template MUST be provided by the submitter,
with a clear description of what the action does.
2. A proposed new action name MUST NOT conflict with any existing
registered action name. A conflict includes a name that
duplicates an existing one, or that appears to be very similar to
an existing one and could be a potential source of confusion.
3. A proposed new action MUST NOT exactly duplicate the
functionality of any existing actions. In cases where the new
action functionality is very close to an existing action, the
designated expert SHOULD clarify whether the submitter is aware
of the existing action, and has an adequate reason for creating a
new action with slight differences from an existing one.
4. If a proposed action is an extension to an existing action, the
changes MUST NOT conflict with the intent of the existing action,
or in a way that could cause interoperability problems for
existing deployments of the protocol.
The IANA registry will contain the name of the action ("Action Name")
and a reference to the section of the specification where the action
registration template is defined ("Reference").
10.1.2. Registration Template for Actions
An action is defined by completing the following template.
Name: The name of the action.
Request-URI Template: The URI template used in HTTP requests for the
action.
Description: A general description of the action, its purpose, etc.
Douglass & Daboo Expires December 31, 2015 [Page 48]

Internet-Draft TZDIST Service June 2015
Index value:: Values in this registry are URNs or URN prefixes that
start with the prefix "urn:ietf:params:tzdist:". Each is
registered independently. The prefix
"urn:ietf:params:tzdist:error:" is used to represent specific
error codes within the protocol as defined in the list of actions
in Section 5 and used in problem reports (Section 4.1.7).
Each registration in the "tzdist Identifiers" registry requires the
following information:
URN: The complete URN that is used or the prefix for that URN.
Description: A summary description for the URN or URN prefix.
Specification: A reference to a specification describing the URN or
URN prefix.
Contact: Email for the person or groups making the registration.
Index Value: As described in [RFC3553], URN prefixes that are
registered include a description of how the URN is constructed.
This is not applicable for specific URNs.
The "tzdist Identifiers" registry has the initial registrations
included in the following sections.
10.4.1. Registration of invalid-action error URN
This section registers the "urn:ietf:params:tzdist:error:invalid-
action" URN in the "tzdist Identifiers" registry.
URN: urn:ietf:params:tzdist:error:invalid-action
Specification: RFCXXXX, Section 5
Repository: http://www.iana.org/assignments/tzdist-identifiers.
Contact: IESG <iesg@ietf.org>
Index value:: N/A.
10.4.2. Registration of invalid-changedsince error URN
This section registers the "urn:ietf:params:tzdist:error:invalid-
changedsince" URN in the "tzdist Identifiers" registry.
URN: urn:ietf:params:tzdist:error:invalid-changedsince
Douglass & Daboo Expires December 31, 2015 [Page 51]

Internet-Draft TZDIST Service June 2015
10. SECDIR: added reference to TLS BCP document.
Changes for -05
1. Now uses its own rules for defining JSON objects.
2. Added new section on time zone versions.
3. Added publisher/version to the list action response meta-data.
4. Changed conditional request and synchronization sections to
better describe how meta-data and data are updated.
5. Added the ability to retrieve leap second information from the
server.
6. Added text to require servers to return all data if a
"changedsince" value is not supported.
7. Switched TZUNTIL to be exclusive rather than inclusive, so that
it now matches the definition of the truncation end point (also
exclusive).
Changes for -04
1. Tweaked invalid-start/end for action expand to indicate outside
truncation range.
2. Added text on use of Accept-Language.
3. Added text on requirement to percent-encode {/tzid}.
4. Moved /observances under /zones{/tzid}.
5. Observances response now includes start/end of actual range
returned if different from what was requested.
6. Truncation end and &end= for get action are now exclusive.
7. Added capabilities action in capabilities example response.
8. Added uri-template items to capabilities action definitions.
9. Added start/end items to the observances response.
10. Error codes are now URNs (with an IANA registration for a tzdist
sub-namespace) and the URNs are used as the type value in JSON
problem reports.
Douglass & Daboo Expires December 31, 2015 [Page 59]